It is known to connect, to electric power networks in shunt connection, static compensators for compensation of the reactive power consumption of the power network and of equipment connected to the power network. One type of such compensators comprises at least one and usually a plurality of thyristor-switched capacitors (TSC). A thyristor-switched capacitor substantially comprises a capacitor in series connection with a controllable semiconductor valve. In addition thereto, an inductive element, an inductor, is usually arranged in series connection with the capacitor to limit the rate of change of the current through the capacitor when the capacitor is connected to the power network and to avoid resonance phenomena with inductive components located in the power network.
The controllable semiconductor valve comprises at least two controllable semiconductor elements, usually thyristors, arranged in anti-parallel connection. By bringing the semiconductor elements in a conducting state, that is, by controlling their firing time relative to the phase position of the voltage of the ac network, the capacitor may be coupled to the power network for generating reactive power. It is to be understood that, in this application, the concept capacitor comprises also those cases where the capacitor is composed of a plurality of mutually connected capacitive elements, sub-capacitors, which are all commonly coupled by the controllable semiconductor valve. Further, it is to be understood that the semiconductor valve may comprise a plurality of mutually series-connected, and then usually pair-wise antiparallel-connected, semiconductor elements, which are each controlled by a firing order. A control device generates individual firing pulses for the semiconductor elements included in the semiconductor valve.
FIG. 1 illustrates a static compensator of the kind described above, which is connected via a transformer TR to an ac network N1. The compensator comprises three capacitors CA, CB, CC, each being shunt-connected to a common voltage busbar BB via a controllable semiconductor valve VA, VB, VC, respectively, and an inductor LA, LB, LC, respectively. The semiconductor valves are schematically illustrated in the figure with two semiconductor elements T1, T2 in antiparallel connection. Control equipment CEQ supplies firing orders COA, COB, COC, respectively, to the semiconductor valves.
For a general description of thyristor-switched capacitors and control thereof, reference is made to, for example, Ake Ekstrom: High Power Electronics HVDC and SVC, Stockholm 1990, in particular pages 10-1 to 10-7.
Since the current through the thyristor-switched capacitor in steady state has a phase position 90 electrical degrees in advance of the voltage across the same, the two antiparallel-connected semiconductor elements of the semiconductor valve should be given firing orders alternately at the times when the time rate of change of the fundamental tone for the voltage across the thyristor-switched capacitor changes sign from a positive value to a negative value, and inversely. If the phase position of the voltage is defined such that, at 0.degree., its amplitude is zero and increasing in a positive direction, under steady-state conditions these sign reversals take place at the electrical angles 90.degree. and 270.degree.. When the above-mentioned time rate of change changes sign from a positive to a negative value, a firing order should be given to that of the semiconductor elements, the conducting direction of which coincides with the expected current direction in the next interval, that is, with the above-mentioned convention, in the interval 90.degree. to 270.degree.. When the mentioned time rate of change again changes signs, a firing order is given to the other semiconductor element, the conducting direction of which coincides with the expected current direction in the interval which is then to follow, that is, with the above-mentioned convention, in the interval 270.degree. to 450.degree..
When the generation of firing orders is brought to an end, for example in dependence on a voltage control system for maintaining the voltage in the ac network or the voltage busbar BB constant, the current through the semiconductor valve will cease at the next zero crossing of the current. The voltage of the capacitor thus remains at a level determined by the voltage of the power network when the current through the capacitor was forced to cease. When a firing order is again generated, according to the criterion mentioned above, and the voltage of the voltage busbar has remained unchanged, the connection of the capacitor occurs, in principle, without any transition phenomena in current and voltage.
Usually, each semiconductor element is associated with an electronic unit with an indicating device which, in some manner known per se, generates indicating signals, indicating that an off-state voltage exists across the semiconductor elements, in the respective conducting direction of the semiconductor elements. Typically, an indicating signal is generated when the off-state voltage amounts to about 50 V across a semiconductor element in the form of a thyristor. These indicating signals are usually transferred from the potential of the semiconductors via light guides to the control equipment arranged at ground potential.
Likewise, in some manner known per se, the control equipment generates, in dependence on received indicating signals, firing orders and supply these to the electronic units, also usually via light guides. In general, therefore, the electronic units comprise circuits with components, for example photodiodes, for transforming the firing order in the form of light into electrical firing signals for each of the semiconductor elements.
To limit current and voltage stresses on the semiconductor elements in connection with a change of their conducting state, a transient protection circuit, a so-called snubber circuit, is usually arranged in parallel connection with the semiconductor elements, this circuit comprising a series connection of resistive and capacitive components.
The above-mentioned functions of the electronic units require electrical energy and the electronic units must therefore have access to a power supply. This power supply should be galvanically separated from ground potential and the electric power should thus be supplied from that ac network to which the thyristor-switched capacitor is connected.
The electronic units usually also comprise a gate circuit which forwards, to the semiconductor elements, firing orders received from the control equipment for firing the respective semiconductor element in dependence on the voltage level of the supply voltage.
A known way of arranging this power supply for thyristor-switched capacitors is illustrated in FIG. 2. The figure schematically illustrates parts of a semiconductor valve of the kind described above, which comprises two thyristors T1, T2 in antiparallel connection, a snubber circuit SC with a snubber capacitor CS and a snubber resistor RS in series connection. Supply devices FD1 and FD2, respectively, are adapted to supply electronic units (not shown in the figure) for the thyristors T1, T2, respectively, with electrical energy. Each one of the supply devices comprises an energy storage in the form of a capacitor, in the figure designated C1 and C2, respectively. The voltage across the capacitors, in the figure designated UF1 and UF2, is supplied to the respective electronic units. A current transformer--not shown in its entirety in the figure--with a primary winding, through which the alternating current through the thyristor-switched capacitor flows, has a number of separate secondary windings, two of which, designated S1 and S2, respectively, are shown in the figure. The supply device FD1 further comprises diodes Da1 and Da2. When current flows through the secondary winding S1, a current path through the supply device FD1 is closed via the diode Da2, the capacitor C1, and via a Zener diode Zc in a supply device FD2', which is adapted for power supply of an electronic unit (not shown) for a thyristor T2', connected in series with the thyristor T2. In the event that the supply device FD2' does not exist, the current path is instead closed via a Zener diode Za' in the supply device FD1. The capacitor C1 is thus supplied with energy via the current through the secondary winding S1.
The thyristor T1 has one anode terminal TA1 and one cathode terminal TC1. When no current flows through the current transformer, that is, when the semiconductor elements are in a non-conducting state, and when the voltage between the anode and cathode terminals exhibits a positive time rate of change, a small amount of energy is supplied to the capacitor C1 through a current path from the anode terminal TA1 via a diode Db1 in the supply device FD2, the snubber circuit SC and a diode D11. Conventionally, the energy storage is designed to contain energy sufficient for the safe function of the electronic unit for a plurality of cycles of the alternating current, which, however, also implies that a plurality of ac cycles are required for supplying, via the snubber circuit, an amount of energy which is large enough for the energy storage to attain a voltage level and an energy content sufficient for the safe function of the electronic unit. Thus, this solution presupposes that the energy requirement of the electronic unit is ensured via supply from a current transformer, which component, of course, complicates and renders more expensive the system for energy supply to the electronic units.
FIG. 3 illustrates a known system for energy supply to electronic units of a corresponding kind in a semiconductor valve included in a converter for conversion between alternating current and high-voltage direct current. A thyristor T1 included in the semiconductor valve has a snubber circuit SC connected between the anode terminal TA1 and the cathode terminal TC1. In this case, the snubber circuit comprises a first series connection of a resistor RS1 and a capacitor CS1, which in turn is connected in series with a second series connection of a resistor RS2 and a capacitor CS2. A third series connection of a capacitor CS3 and a resistor RS3 is connected between the point of connection between the above-mentioned capacitors and a supply device FDH for energy supply of an electronic unit (not shown in the figure) for the thyristor T1. When the voltage between the anode and cathode terminals exhibits a positive time rate of change, a current path is formed from the anode terminal via the first and third series connections, a diode D11 in the supply device and an energy storage in the form of a capacitor C1h to the cathode terminal. The voltage across the capacitor, designated UF1h in the figure, is supplied to the electronic unit. In this case, the energy storage is designed to be charged during each cycle, via the current path mentioned, with an amount of energy which is large enough for the energy storage to attain a voltage level and an energy content sufficient for the safe function of the energy unit during one cycle of the alternating current. In this case, the snubber circuit is designed as a voltage-divider circuit, which implies that only part of the current through the snubber circuit is supplied to the supply device.